THE EARTH'S SURFACE ICE ACTION. 65 



at first must be large ; but after the disintegrated 

 and loosened rocks are removed, the effects of the 

 ice must gradually lessen, as its own polishing of 

 the rocks it passes over destroys its opportunity of 

 exercising its abrading powers. That this is the 

 case seems proved by two, and in places, even three 

 or four, distinct systems of striae being preserved on 

 one and the same rock-surface; these having been 

 cut evidently by ice-streams which flowed at differ- 

 ent times in different directions, and sometimes 

 even in nearly opposite courses. In West Galway 

 there are various systems of striae, some going west- 

 ward, others south-east or north-west, and on one 

 rock-surface two or three different movements of the 

 ice are recorded. 1 The newer stria3 must have been 

 engraved by an ice-stream of considerable magnitude, 

 which probably existed for a long time. If a glacier 

 is capable of much denudation, all previous rock 

 markings, especially such small ones as striae not 

 the tenth of an inch in depth, should be obliterated 

 by it. Nevertheless, this is not the case, but in 

 some localities the oldest striae are so distinct that 

 the relative ages of striae on one rock-surface might 

 be questionable, were it not that in other places 

 their true ages were proved. It may be asked, If 



1 " General Glaciation of lar-Connaught and its Neighbourhood, 

 with a Map." By G. H. Kinahan and M. H. Close. Hodges & Foster, 

 Dublin. 



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